1. Field of the Invention
This invention relates to a method of operation of a liquid crystal light valve for use in large screen projection and the like.
2. Description of the Prior Art
Typically, a liquid crystal light valve is made up of several layers of material with a light blocking layer and a mirror located between a photoconductor and a liquid crystal. The light blocking layer is located between the photoconductor and the mirror. On what would otherwise be the outer surfaces cf the photoconductor and the liquid crystal, there are located transparent contacts. On what would otherwise be the outer surfaces of the transparent contacts, there are located layers of glass that are optically correct.
Liquid crystal light valves are operated in a reflecting mode and a mirror is used to reflect the front illumination. Unfortunately, the mirror is not completely reflecting. As a result, some of the very high intensity light leaks through to the semiconductor and produces carriers. These carriers change the resistivity of the semiconductor. To prevent this change in resistivity, a light blocking layer is employed between the mirror and the semiconductor. The light blocking layer introduces extra resistance, capacitance and complexity to the device. Further, when liquid crystal light valves are used for large screen projection and similar applications, the photoconductor film must be fairly thick (usually greater than five micrometers) to reduce the capacitance of the film layer and to provide a sufficiently large dark resistance. Capacitance is proportional to the inverse of the thickness. {See "1972 SID International Symposium Digest of Technical Papers", June 1972, pages 70-71 (D1); U.S. Pat. No. 4,037,932 (D2); and "Nouvelle Revue D'Optique appliqee", July/August 1971, vol. 2, no. 4, pages 221-228 (D3)}
When light is shone on the back surface of the photoconductor, most of the light is absorbed in the first few hundred nanometers or less (see the lower curve in FIG. 1). The carriers produced by this light remain near the surface of the photoconductor and produce a very small change in the overall resistance of the photoconducting layer and therefore a very small change in the signal to the liquid crystal (i.e. low sensitivity).
Several methods have been advanced to move the carriers into the bulk of the photoconductor by applying internal or external electric fields. An AC field must be applied to the liquid crystal in order to prevent the transparent contacts from being electrochemically removed and the photoconductor must have both carrier types active. If the photoconductor is unipolar, then the carriers are swept into the bulk of the semiconductor for one bias polarity and held at the surface for the other bias polarity. In other words, the unipolar photoconductor only works properly for one bias polarity. Another problem that arises from sweeping the carriers from the surface into the bulk of the photoconductor is that the semiconductor or photoconductor must be optimized for long carrier lifetimes to allow enough time for the carriers to be swept through the bulk of the semiconductor or photoconductor. This can place severe restrictions on the frame rates which can be achieved.